U.S. patent number 4,623,883 [Application Number 06/680,118] was granted by the patent office on 1986-11-18 for automatic communications switch.
This patent grant is currently assigned to NCR Corporation. Invention is credited to Noel R. Konen.
United States Patent |
4,623,883 |
Konen |
November 18, 1986 |
Automatic communications switch
Abstract
An automatic communications switching circuit is used to couple
a primary terminal, an alternate terminal, a cluster of secondary
terminals and a host system to enable communication and data
transfer among the terminals and host system. The primary terminal
normally handles data transfer and communication for the cluster of
secondary terminals when the switching circuit is in the first
state. A lack of polling by the primary terminal (which is an
indication of inoperativeness of the primary terminal) is sensed by
the alternate terminal which then starts the polling process. The
polling by the alternate terminal is sensed by the switching
circuit which is then switched to a second state in which the
alternate terminal functions as the "primary" terminal.
Inventors: |
Konen; Noel R. (Easley,
SC) |
Assignee: |
NCR Corporation (Dayton,
OH)
|
Family
ID: |
24729747 |
Appl.
No.: |
06/680,118 |
Filed: |
December 10, 1984 |
Current U.S.
Class: |
340/2.9; 370/221;
700/82; 714/25 |
Current CPC
Class: |
H04L
12/00 (20130101); G06F 13/22 (20130101) |
Current International
Class: |
G06F
13/22 (20060101); G06F 13/20 (20060101); H04L
12/00 (20060101); H04Q 001/00 () |
Field of
Search: |
;340/825.01 ;364/187
;371/9,65 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yusko; Donald J.
Attorney, Agent or Firm: Hawk, Jr.; Wilbert Sessler, Jr.;
Albert L. Wargo; Elmer
Claims
What is claimed is:
1. A system comprising:
a primary terminal having primary and secondary communication
lines;
an alternate terminal having primary and secondary communication
lines;
a cluster of terminals having communication lines to interconnect
said cluster of terminals;
a switching circuit being operatively coupled to said primary and
alternate terminals and said communication lines to said cluster of
terminals; said switching circuit also being switchable between
first and second states;
said switching circuit having first means for coupling said primary
communication lines of said primary terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said first state and also for coupling said
secondary communication lines of said primary terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said second state;
means for sensing when said primary terminal is inoperative;
and
said switching circuit also having means for switching
automatically said switching circuit from said first state to said
second state when said sensing means senses that said primary
terminal is inoperative;
said sensing means comprising means coupled to said secondary
communication lines of said alternate terminal to check for polling
signals by said primary terminal and in which a lack of said
polling signals is indicative of said primary terminal being
inoperative;
said switching means comprising:
means associated with said alternate terminal for producing polling
signals on said primary communication lines of said alternate
terminal; and
actuating means responsive to said polling signals on said primary
communication lines of said alternate terminal to switch said
switching circuit from said first state to said second state.
2. A system comprising:
a primary terminal having primary and secondary communication
lines;
an alternate terminal having primary and secondary communication
lines;
a host processor;
a cluster of terminals having communication lines to interconnect
said cluster of terminals;
a switching circuit being operatively coupled to said primary and
alternate terminals, said host processor, and said communication
lines to said cluster of terminals, and said switching circuit also
being switchable between first and second states;
said switching circuit having first means for coupling said primary
communication lines of said primary terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said first state and also for coupling said
secondary communication lines of said primary terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said second state;
said switching circuit having second means for coupling said
secondary communication lines of said primary terminal with said
host processor when said switching circuit is in said first state
and also for coupling said secondary communication lines of said
alternate terminal with said host processor when said switching
circuit is in said second state;
said switching circuit having third means for coupling said
secondary communication lines of said alternate terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said first state and also for coupling said
primary communication lines of said alternate terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said second state;
means for sensing when said primary terminal is inoperative;
and
said switching circuit also having menns for switching
automatically said switching circuit from said first state to said
second state when said sensing means senses that said primary
terminal is inoperative;
said sensing means comprising means coupled to said secondary
communication lines of said alternate terminal to check for polling
signals by said primary terminal and in which a lack of said
polling signals is indicative of said primary terminal being
inoperative,
said switching means comprising:
means associated with said alternate terminal for producing polling
signals on said primary communication lines of said alternate
terminal; and
actuating means responsive to said polling signals on said primary
communication lines of said alternate terminal to switch said
switching circuit from said first state to said second state.
3. The system as claimed in claim 2 in which said first, second,
and third coupling means include first, second, and third relays,
respectively, with each of said first, second, and third relays
having an operating coil; and
said actuating means includes means for energizing said operating
coils of said first, second, and third relays to switch said
switching circuit from said first state to said second state in
response to said polling signals.
4. The system as claimed in claim 3 in which said actuating means
includes a transformer coupling between the primary lines of said
alternate terminal and said energizing means.
5. The system as claimed in claim 4 in which said switching circuit
has means for indicating whether said switching circuit is in said
first state or said second state.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to a system in which a primary
terminal normally handles data interchange or communications among
a plurality of secondary terminals and in which an alternate
terminal is provided and has as one of its functions that of acting
as an "alternate primary terminal" to handle the communications
among the plurality of secondary terminals, for example; this
invention relates specifically to a communications switch which
automatically enables the alternate primary terminal to function as
the primary terminal when the primary terminal is inoperative for
one reason or another.
One of the known ways in which the switching from the primary
terminal to the alternate primary terminal was effected in a system
as generally described was to wait until the primary terminal was
inoperative as indicated by, for example, a display message
indicating the inoperative state on one of the secondary terminals.
An operator at the secondary terminal would have to go to the
primary terminal which might be located in a supervisor's office in
order to find out what was wrong with the system. The primary
terminal might indicate, through its associated display, that it
was unable to communicate with the secondary terminal. Thereafter,
the operator would actuate a manually-operated switch which caused
the alternate primary terminal to function as the primary terminal
and the "now downed primary terminal" to function as the alternate
primary terminal to permit communications to be resumed.
SUMMARY OF THE INVENTION
A preferred embodiment of this invention is disclosed in a system
comprising: a primary terminal having primary and secondary
communication lines; an alternate terminal having primary and
secondary communication lines; a cluster of terminals having
communication lines to interconnect said cluster of terminals; a
switching circuit being operatively coupled to said primary and
alternate terminals and said communication lines to said cluster of
terminals; said switching circuit also being switchable between
first and second states; said switching circuit having first means
for coupling said primary communication lines of said primary
terminal with said communication lines to said cluster of terminals
when said switching circuit is in said first state and also for
coupling said secondary communication lines of said primary
terminal with said communication lines to said cluster of terminals
when said switching circuit is in said second state; said switching
circuit having second means for coupling said secondary
communication lines of said alternate terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said first state and also for coupling said
primary communication lines of said alternate terminal with said
communication lines to said cluster of terminals when said
switching circuit is in said second state; means for sensing when
said primary terminal is inoperative; and said switching circuit
also having means for switching automatically said switching
circuit from said first state to said second state when said
sensing means senses that said primary terminal is inoperative.
The communications switch of this invention provides a simple, low
cost way of automatically sensing when a controlling primary
terminal is inoperative and to automatically switch to an alternate
controlling terminal when the primary terminal is inoperative. The
communications switch also enables the primary terminal and the
alternate controlling terminal to communicate with a host
system.
These advantages and others will be more readily understood in
connection with the following description, claims, and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram, in block form, showing a system in
which a preferred embodiment of the communications switch of this
invention may be used, with the switch being shown in a first
state;
FIG. 2 is a schematic diagram, in block form, showing only a
portion of the system shown in FIG. 1, with the switch being shown
in the second state of two states;
FIG. 3 is a schematic diagram showing the contacts of switching
relays associated with the communications switch shown in FIG. 1;
and
FIG. 4 is a schematic diagram of a circuit which is used to switch
the communications switch between the first and second states
mentioned.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic diagram, in block form of a system 10 in
which the communications switch (designated generally as 12) of
this invention may be used. The system 10 includes a plurality of
data entry terminals 14-1, 14-2, 14-3 and 14-4, for example, which
are coupled to a primary DLC terminal 16 and an alternate DLC
terminal 18 through the communications switch 12, hereinafter
referred to as switch 12. The term DLC used with terminal 16, for
example, is an acronym which stands for Data Link Communications
which is a communications protocol used by NCR Corporation for
"in-house" communications. This DLC protocol is currently available
from the NCR Corporation of Dayton, Ohio. The switch 12 also
provides for coupling to an external entity such as a host
processor 20 via a link 19 and an interface 22 such as a modem.
In the embodiment described, the primary terminal 16 and the
alternate terminal 18 may be conventional terminals like the NCR
2951, 2950, or 2957. Similarly, the terminals 14-1, 14-2, 14-3, and
14-4 may be conventional terminals like the NCR 2950, for example,
and the host processor may be conventional like the NCR 5094. All
the NCR products mentioned in this paragraph are available from the
NCR Corporation of Dayton, Ohio.
In the system 10 shown in FIG. 1, the primary terminal 16 may be
considered as the "host" for the terminals 14-1 through 14-4. In
general, the terminals, like 14-1, may be terminals like those
found in grocery stores, for example, which are coupled together
over a communication link 24 to the primary terminal 16. Under
normal situations, the primary terminal 16 handles the data
interchange among the terminals 14-1 through 14-4. When these
terminals, like 14-1, want to communicate with the host processor
20, they do so by communicating with the primary terminal 16 which,
in turn, communicates with the host processor 20. When the primary
terminal 16 "goes down" or is unable to function as described, the
alternate terminal 18 becomes the "primary" terminal to handle the
communications among the terminals 14-1 through 14-4 and to
communicate with the host processor 20. When the alternate terminal
18 becomes the "primary" terminal, the primary terminal 16 becomes
the "alternate" terminal. The switching of the functions of the
terminals 16 and 18 is effected by the communications switch 12
under certain circumstances as will be described hereinafter.
Before describing the communications switch 12, it appears
appropriate to discuss generally the known terminals 16, 18, and
14-1 through 14-4. The primary terminal 16 comprises a read only
memory (ROM) 26, a random access memory (RAM) 28, a processor (MP)
30, a keyboard (KB) 32, a display or cathode ray tube (CRT) 34, a
printer 36, a data storage disc 38, a communication interface 40,
and interface and control logic 42 which couple the various
components mentioned. The alternate terminal 18 has components
which are identical to those in terminal 16, and accordingly, the
corresponding elements in alternate terminal 18 are given the same
reference numerals as are given the elements of terminal 16;
however, these reference numerals have a dash number (-1) attached
to them.
The terminal 14-1 (FIG. 1) includes a ROM 44, RAM 46, processor
(MP) 48, keyboard KB (50), a display or cathode ray tube (CRT) 52,
a communication interface 54, and interface and control logic 56
which couple the various components mentioned. The terminals 14-1
through 14-4 are all identical, except for a few differences to be
explained, and accordingly, the identical elements are provided
with a reference numeral having a dash number (-1) as explained in
the previous paragraph. Terminal 14-2 is different because it
includes a printer 58, and terminal 14-3 is different because is
includes a weighing scale 60.
The terminals 14-1 through 14-4 (FIG. 1) are sometimes referred to
as secondary terminals in that they are controlled in large part by
a higher-order terminal like primary terminal 16. These terminals
14-1 through 14-4 do not have large memories, and consequently,
they often have to transfer data to the data storage disc 38
associated with the primary terminal 16. The primary terminal 16
also is used to control some of the functions at the terminals 14-1
through 14-4. For example, an operator at terminal 14-1 may wish to
use the printer 58 associated with terminal 14-2 or the weighing
scale 60 associated with terminal 14-3. The request made by an
operator at the terminal 14-1 is transferred to the primary
terminal 16 which then checks the availability of the scale 60, for
example, at terminal 14-3. If the scale 60 is available, the
primary terminal 16 issues an order or control signal to energize
the scale 60. The operator then uses the scale 60 and the weight
from the scale 60 is then routed to the primary terminal 16 which
thereafter indicates the weight on the CRT 52 of the terminal 14-1
on which the request for the use of the scale 60 was entered. The
primary terminal 16 may also be used as a "super accountant" to
accumulate data from the terminals 14-1 through 14-4, to generate
reports from the data, and to transfer the data or reports
generated thereby to the host processor 20.
With the general discussion given relative to the operation of the
system 10 (FIG. 1), it is apparent that it is necessary to maintain
data transfer among the various terminals and the host processor 20
shown in FIG. 1. As previously stated, the alternate terminal 18
takes over in place of the primary terminal 16 when terminal 16
becomes inoperative. In this regard, the primary terminal 16 is
coupled to the communications switch 12 by primary DLC lines which
are marked P-P in FIG. 1, and secondary DLC lines which are marked
P-S. Correspondingly, the alternate terminal 18 is coupled to the
communications switch 12 by primary DLC lines which are marked as
A-P in FIG. 1, and secondary DLC lines which are marked A-S.
In the embodiment described, the P-P and the P-S lines of the
primary terminal 16 (FIG. 1), and the A-P and A-S lines of the
alternate terminal 18 are each comprised of four lines or
conductors (two for transmitting and two for receiving) as will be
described hereinafter. The communications link 24 is also comprised
of four conductors with two conductors being used for the
transmitting mode and two being used for the receiving mode.
The general connections through the communications switch 12 (FIG.
1) are best explained by viewing schematically, the connections
effected by the switch 12 when it is in either its first or second
state. When the switch 12 is in the first state shown in FIG. 1,
the P-P lines from the primary terminal 16 pass through the switch
to be coupled to the communication link 24 and the terminals 14-1
through 14-4, and in addition, the P-P lines are coupled to the A-S
(secondary lines) of the alternate terminal 18. Notice that the
secondary lines P-S of the primary terminal 16 are coupled to the
host processor 20 when the switch 12 is in the first state which is
the normal operating position. When the switch 12 is in the first
state, the primary terminal 16 acts as the controlling host or
terminal for the secondary terminals 14-1 through 14-4. Note also
that while the alternate terminal 18 is presently in the back-up
mode in the example described, it is also coupled to the primary
terminal 16 (via the A-S lines) and consequently, it can also
perform operations instead of simply sitting idly in a back-up
mode.
The general connections through the communications switch when it
is in the second state are shown schematically in FIG. 2. In this
state, the alternate terminal 18 becomes the "primary" terminal,
and the primary terminal 16 becomes the "alternate" terminal.
Notice that the communication link 24 is now coupled through the
switch 12 to the A-P or primary lines of the alternate terminal 18
and the secondary lines P-S of the primary terminal 16, and the
alternate terminal's secondary lines A-S are coupled to the host
processor 20. Notice also that the P-S or secondary lines from the
primary terminal 16 are coupled through the communications link 24
to the primary lines A-P of the alternate terminal 18.
Having described the general connections through the communications
switch 12, it now appears appropriate to discuss the details of
this switch which are shown in FIGS. 3 and 4.
FIG. 3 is a schematic diagram showing the contacts of switching
relays that are associated with the communications switch 12. In
the embodiment described, there are three relays, namely relay 1,
relay 2, and relay 3 whose coils 61, 62, and 63, respectively, are
shown in FIG. 4. The contacts of the relays 1, 2, and 3 are shown
in their normal operating condition in FIG. 3, in which condition
the coils 61, 62, and 63, respectively, are in the deenergized
state; in this state, the communications switch 12 is in the state
shown in FIG. 1.
In order tp simplify the drawing in FIG. 3, the contacts associated
with a particular relay are encircled by a dashed line as shown for
relays 1, 2, and 3. In the embodiment described, each of the relays
1, 2, and 3 is a four pole, double-throw switch. Accordingly, relay
1 has terminal position contacts which are numbered 1-1 and 1-2 and
the associated moveable arm is numbered 1-3 for each pole. Only one
set of contacts 1-1 and 1-2 and one moveable arm 1-3 is marked
within the dashed line marked as relay 1 in FIG. 3 in order to
simplify the drawing. The same numbering technique is used for
relay 2 which has terminal position contacts 2-1 and 2-2 and the
associated moveable arm 2-3. Similarly, the same numbering
technique is used for relay 3 which has terminal position contacts
3-1 and 3-2 and the associated moveable arm 3-3.
The contacts shown in FIG. 3 are assigned designations which make
their correlation with the contacts in switch 12, shown only
schematically in FIG. 1, easy to follow. For example, the line
marked P-P (coming from the primary terminal 16 in FIG. 1) is
comprised of four lines marked PPT1, PPT2, PPR1, and PPR2 in FIG.
3. The PPT1 and PPT2 lines comprise a pair of "transmit" lines and
the PPR1 and PPR2 lines comprise a pair of "receive" lines.
Similarly, the secondary lines P-S from the primary terminal 16 are
comprised of the four lines marked PSR1, PSR2, PST1, and PST2 in
FIG. 3. The PSR1 and PSR2 lines comprise the "receive" lines and
the PST1 and PST2 lines comprise the "transmit" lines. The primary
lines A-P (FIG. 1) coming from the alternate terminal 18 are
comprised of four lines marked APT1, APT2, APR1, and APR2 as shown
in FIG. 3. The APT1 and APT2 lines comprise a pair of "transmit"
lines and the APR1 and APR2 lines comprise a pair of "receive"
lines. The secondary lines A-S (FIG. 1) coming from the alternate
terminal 18 are comprised of four lines marked ASR1, ASR2, AST1,
and AST2 as shown in FIG. 3. The ASR1 and ASR2 lines comprise a
pair of "receive" lines and the AST1 and AST2 lines comprise a pair
of "transmit" lines. The link 19 to the host processor 20 (FIG. 1)
is comprised of four lines HT1, HT2, HR1 and HR2 as shown in FIG.
3. The HT1 and HT2 lines comprise a pair of "transmit" lines and
the HR1 and HR2 lines comprise a pair of "receive" lines as shown
in FIG. 3. And finally, the link 24 leading to the terminals 14-1
through 14-4 is comprised of the lines TR1, TR2, TT1 and TT2. The
TR1 and TR2 lines comprise the "receive " lines, and the TT1 and
TT2 lines comprise the "transmit" lines. These lines TR1, TR2, TT1,
and TT2 are connected to each of the terminals 14-1 through 14-4.
Indentification or address codes which are part of a transmitted
message are used to select the particular terminal 14-1 through
14-4 which is to receive the message. Notice that a message
transmitted from the primary terminal 16, passes over the transmit
lines PPT1 and PPT2 (FIG. 3) to the receive contacts TR1 and TR2 of
the terminals 14-1 through 14-4, and in addition, the message is
transmitted through the arms 3-3 and contacts 3-1 of relay 3 to the
secondary receive contacts ASR1 and ASR2 of the alternate terminal
18. Notice that the pair of primary "transit lines" PPT1 and PPT2
(FIG. 3) of the primary terminal 16 are aligned with the secondary
"receive" lines PSR1 and PSR2 across the contacts of relay 1.
Similarly, the primary "receive" lines PPR1 and PPR2 are aligned
with the secondary "transmit" lines PST1 and PST2 across the
contacts of relay 1. In other words, when the switch 12 is in the
first state shown in FIGS. 1 and 3, the primary lines PPT1, PPT2,
PPR1, AND PPR2 are active to the terminals 14-1 through 14-4 and to
the secondary lines ASR1, ASR2, AST1 and AST2, respectively of the
alternate terminal 18. This enables the alternate terminal 18 to
monitor the activity on the transmission lines PPR1 and PPT2 of the
primary terminal 16 via the secondary lines ASR1 and ASR2
associated with the alternate terminal 18. This is how the
alternate terminal 18 determines when the primary terminal 16 is
not functioning properly so as to cause the communications switch
12 to switch to the second state to enable the alternate terminal
18 to function as the primary terminal.
In the normal operating situation, the communications switch 12 is
in the first state shown in FIGS. 1 and 3. In the first state
mentioned, the primary terminal 16 polls the terminals 14-1 through
14-4 and because of the connection shown in FIG. 3 as previously
mentioned, the alternate terminal 18 also monitors the polling by
its ASR1 and ASR2 lines. The primary terminal 16 polls the
terminals 14-1 through 14-4 and the alternate terminal 18 to
ascertain which of them has data to communicate to the primary
terminal 16 or has data to transfer to one another. The terminals
14-1 through 14-4 communicate with the host processor 20 through
the primary terminal 16.
In the embodiment described, the primary terminal always polls the
terminals 14-1 through 14-4 (between data transfer activity) as a
part of normal operation, and when the primary terminal does not
poll, it means that something is wrong with the primary terminal
16. There is software associated with the alternate terminal 16
which constantly monitors its ASR1 and ASR2 lines to determine
whether or not the primary terminal 16 is polling the terminals
like 14-1 through 14-4. When the alternate terminal 18 determines
that the primary terminal 16 is not polling and is therefore
inoperative, the alternate terminal 18 (through its associated
software) then starts to send out polling signals on its primary
transmit lines APT1 and APT2. The lines APT1 and APT2 are tapped at
contacts A and B in FIG. 3, and these contacts provide the input to
the rest of the communications switch 12 shown in FIG. 4.
The portion of the communications switch 12 shown in FIG. 4
contains that portion which does the actual switching of the relays
1, 2, and 3 from the first state to the second state in response to
polling by the alternate terminal 18 as just described. Before
discussing the operation of the switch 12, it appears appropriate
to discuss some of its components.
A preferred embodiment of the communications switch 12 is shown in
FIGS. 3 and 4, and the values of certain components included
therein are shown in FIG. 4. These components include a
conventional full-wave, rectifier bridge circuit 64 which includes
a step down transformer T1 which transforms 120 volts A.C. to 12.6
volts A.C. (RMS) and a full-wave, rectifier bridge circuit 66 to
provide a D.C. output of 17.13 volts across the conductors 68 and
70. Capacitor C1, which is connected across the conductor 68 and
70, is used to eliminate "spikes" and to provide a stable or
"hold-up" voltage. A conventional regulator VR1 is connected across
the conductor 68 and 70 to provide a 5 volt D.C. output on
conductor 72. The regulator VR1 is a conventional regulator such as
#7805 which is manufactured by Texas Instruments, Inc., for
example. Capacitor C2, which is connected between conductor 72 and
ground, is used to provide a "hold-up" voltage during the switching
of transistor Q1. Capacitor C4 is used to eliminate "spikes" as is
conventionally done. Resistor R1, which has one end thereof
connected to conductor 68, is a current limiting resistor which is
used to limit the current to 240 milliamperes and to drop the
voltage to about 12 volts D.C. in the embodiment described. The
resistor R2 provides some constant load in the circuit. The other
end of resistor R1 provides the +D.C. voltage on conductor 74 to
the relay coils 61, 62, and 63. A capacitor C7, connected between
the resistor R1 and ground, eliminates any "spikes" which might
exist at this point. The voltage on conductor 74 does not need to
be regulated critically. In the embodiment described, the relay
coils 61, 62, and 63 take about 9.6 to 18 volts when energized.
The plus (+) end of each of the relay coils 61, 62, and 63 (FIG. 4)
is connected to the positive voltage on conductor 74 as shown. The
negative (-) end of each of the relay coils 61, 62, and 63 is
connected to the collector of transistor Q2 by a conductor 76. When
the transistor Q2 conducts (as will be described hereinafter), the
coils 61, 62, and 63 are energized to switch the communications
switch 12 to the second state as previously described. The diodes
D7, D8, and D9 are placed across the (+) and (-) ends of coils 61,
62, and 63, respectively, to provide a discharge path for the
"inductive kick" which is generated when these coils are
de-energized. The transistor Q2 is a power transistor which is
capable of handling the current loads for a particular application.
In the embodiment described, transistor Q2 is transistor 2N3055,
which is manufactured, for example, by Texas Instruments, Inc. of
Dallas, Texas.
The regulated +5 D.C. voltage from the regulator VR1 that appears
on the conductor 78 provides the voltage for the voltage-divider
resistors R4 and R5 which maintain the voltage on the base of
transistor Q1 at 0.5 volts which is about 0.1 or 0.2 volts below
the turn-on voltage of transistor Q1. The emitter of transistor Q1
is connected to ground, and its collector is connected to the +5
volts on conductor 78 through a resistor R3. When the transistor Q1
is in a non-conducting state, the voltage on conductor 80
(connected to the collector of Q1) is slightly less than 5 volts,
and when the transistor Q1 conducts, the voltage on conductor 80
drops close to the ground level. The capacitor C5, which is
connected between the base of transistor Q1 and ground, provides
"spike" protection as previously explained.
The conductor 80 coming from the collector of transistor Q1 (FIG.
4) is fed into pins 1 and 2 of unit 82 and is also fed into pins 4
and 5 of unit 84. Units 82, 84, and 86 are part of an integrated
circuit pack #74LS00 which is manufactured by Texas Instruments,
Inc., for example. The units 82, 84, and 86 are used as inverters
in the embodiment described. The pin 3 output from unit 82 is
connected to the base of transistor Q2 via a resistor R6. The
capacitor C6 is connected between the base of transistor Q2 and
ground for "spike" protection as previously mentioned. The units 84
and 86 are used to control indicators as will be described
hereinafter.
As previously stated, the contacts A and B shown in FIG. 3 provide
the input to the portion of the communications switch 12 shown in
FIG. 4. The contacts A and B pick up the polling signals being
outputted by the alternate terminal 18 (over the lines APT1 and
APT2) when the alternate terminal 18 begins to function as a
primary terminal as previously described.
The polling signals presented at contacts A and B in FIG. 4 are fed
into the primary winding of transformer T2, with the resistor R9
being placed in series with the primary winding of the transformer
T2 to provide an appropriate impedance matching. The transformer T2
is used to isolate the portion of the switch 12 shown in FIG. 4
from the rest of the circuit, and it is also used to provide a
voltage change when passing through the transformer T2. The actual
absolute voltage of the rectified current coming out of the
transformer T2 is not important per se. The secondary of the
transformer T2 (FIG. 4) is connected to a conventional full-wave,
rectifier bridge circuit 88 which is comprised of diodes D1, D2, D3
and D4 as shown. A capacitor C3 is placed across the output of the
bridge circuit 88, with the positive (+) D.C. voltage appearing on
conductor 90. The capacitor C3 is charged by the output of the
bridge circuit 88, and this capacitor provides a stable D.C.
voltage to the base of transistor Q1 via the current-limiting
resistor R10.
The operation of the portion of the communications switch 12 shown
in FIG. 4 is as follows. When the polling appears on contacts A and
B, the polling current is rectified and the resulting D.C. voltage
is used to charge the capacitor C3 and to provide a D.C. voltage to
the base of transistor Q1. When the voltage on the base of
transistor Q1 rises to about 0.6 to 0.7 volts, in the embodiment
described, the transistor Q1 turns on, causing the voltage at its
collector to fall from about five volts to about zero volts or
"ground". When the voltage on conductor 80 drops to about zero
volts, the output of unit 82 (which functions as an inverter) is
switched to a high-enough, positive voltage to turn transistor Q2
on by applying the output voltage from unit 82 through the resistor
R6 to the base of transistor Q2. When transistor Q2 is turned on,
its collector and conductor 76 are placed at ground level to
thereby effectively energize the coils 61, 62, and 63 causing the
communications switch 12 to switch from the first or normal state
shown in FIGS. 1 and 3 to the opposite or second state shown
schematically in FIG. 2. In the second state, the alternate
terminal 18 functions as the primary terminal as previously
described. After repairs are made to the primary terminal 16 to
restore it to an operative condition, the alternate terminal 18 is
reset or turned off momentarily; this will cause the switch 12 to
be restored to the first state and also will cause the primary
terminal 16 to assume control again. If the primary terminal 16 is
still inoperative, the switch 12 will be switched automatically to
the second state in which the alternate terminal 18 is in
control.
In order to provide an indication as to which state (first or
second) the communications switch 12 is in, display lights D5 and
D6 (FIG. 4) are used. When the switch 12 is in the first or normal
state, there are no polling signals at contacts A and B, and
consequently transistor Q1 is turned off, and the voltage on
conductor 80 is near five volts. A high level input to pins 4 and 5
of unit 84 causes the output pin 6 to invert and fall to a low
level. The low level from pin 6 that is fed into input pins 9 and
10 of unit 86 is inverted thereby, causing the output pin 8 of unit
86 to rise to a high level which thereby energizes the display
light D6 to indicate that the primary terminal 16 is functioning
normally. When the alternate terminal 18 is functioning as the
primary terminal, the voltage level on conductor 80 is at low or
ground level. A low voltage level at pins 4 and 5 of unit 84 causes
its output pin 6 to rise to a high voltage level which energizes
display light D5.
Some miscellaneous comments appear in order. Certain resistors are
placed across certain primary "transmit" and "receive" lines to
provide a dummy load to prevent radio frequency interference (RFI)
and to terminate the lines; these resistors comprise: resistor R15
(FIG. 3) which is placed across the lines PPT1 and PPT2; resistor
R10 which is placed across the lines PPR1 and PPR2; resistor R11
which is placed across the HT1 and HT2 lines; resistor R12 which is
placed across the HR1 and HR2 lines; resistor R13 which is placed
across the APT1 and APT2 lines; and R14 which is placed across the
APR1 and APR2 lines. While the embodiment of the switch 12
described in FIGS. 3 and 4 has been described as having the relays
1, 2, and 3 de-energized when the switch 12 is in the first state
mentioned, the switch 12 could be modified to have the relays 1, 2,
and 3 energized when in the first state and de-energized when the
switch 12 is to be operated in the second state.
* * * * *